A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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A doubt regarding Black Hole Complementarity

A friend was explaining Black Hole Complementarity to me, and at one point he said that to get a (horrendously) mixed quantum state, i.e. a thermal density matrix without a heat bath, one takes a ...
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How do we know the Schwarzschild solution contains an object of mass $M$?

The Schwarzschild metric is $$ds^2 = - \left( 1 - \frac{2GM}{r} \right) dt^2 + \left(1-\frac{2GM}{r}\right)^{-1} dr^2 + r^2 d\Omega^2.$$ In Carroll's GR book, it is claimed that $M$ is the mass of the ...
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Why would a black hole evaporate? [duplicate]

Hawking radiation doesn't make sense to me, with respect to black holes getting smaller. It would seem that any particle (or anti-particle) leaving the Schwartzchild radius would have a similar ...
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26 views

Non equilibrium black hole radiating away energy and angular momentum, with total energy and angular momentum conserved

The question refers to whether mass (i.e., energy) and angular momentum can be considered to have been carried away by the gravitational radiation in the hole settling down. And whether those entities ...
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Pair Production via Black Hole

I think this should be a straight forward question. Is it possible for a photon to pass near a black hole and be turned into a matter and anti-matter pair according to current theory? Edit: From the ...
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1answer
58 views

Are black holes hollow? [closed]

So when enough matter to create a black hole falls towards the center of its collective mass, at one point the mass/area ratio becomes high enough to form the event horizon of the black hole. Because ...
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1answer
46 views

Black hole gravity vs parent star gravity

In the cases of black holes that form from supernova and collapse of a massive star, I understand that in most of these cases, the star loses significant amounts of mass from the explosion. ...
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62 views

Utilising Black Holes as a potential energy source

I'm aware of the Penrose process and the basic physics behind that. Also, I know that the Blandford-Zjanek process (That is potentially responsible for the relativistic jets). Aside from these two, ...
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If Black Holes Consume Space What Is The Space Traversing? [duplicate]

I've heard it stated by physicists that space moves into (Or through/down into if you like) black holes faster than the speed of light and that this is possible because the speed of light is a ...
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493 views

An explanation of Hawking Radiation

Could someone please provide an explanation for the origin of Hawking Radiation? (Ideally someone who I have been speaking with on the h-bar) Any advanced maths beyond basic calculus will most ...
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1answer
33 views

If black hole is a “hole”, how can its position be pinpointed from every position in universe? [closed]

Ok, this is going to sound lame but, here goes: When we say that a black hole is situated at a distance of 1 light year(let) FROM EARTH, that means it may situated at a distance of 100 light years ...
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1answer
33 views

Eddington-Finkelstein coordinate

The Eddington-Finkelstein coordinates in case of Schwarzschild metric are defined as \begin{align} u&=t-r^*\\ v&=t+r^* \end{align} where $$r^*=r+2GM\ln\left|\frac{r}{2GM}-1\right|$$ The ...
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what is the metric of N-sheeted $AdS_3$?

Suppose the AdS$_3$ metric is given by $$ ds^2 =d\rho^2+cosh^2\rho d\psi^2 +sinh^2 \rho d\phi^2 $$ what is the n-sheeted space of it? Can the n-sheeted BTZ be constructed from it by identifications ...
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1answer
91 views

Gravitational waves from neutron star - neutron star merger

How is neutron star-neutron star merger different from Black Hole-Black Hole merger and what information can be extracted from gravitational waves emitted by these processes?
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127 views

Is this analogy of Hawking Radiation correct?

Through reading of textbooks and other research papers, I have settled on the analogy of hawking radiation below (Written completely by myself) Within the ergosphere of the black hole, virtual ...
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1answer
58 views

Can electrons escape a black hole?

Assuming that when an electron that changes energy states in an atom, and moves to a different orbit around the nucleus, but does not move through the space between orbits when it changes states, an ...
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1answer
57 views

Big Bang not really the beginning of a completely new universe? [closed]

From what I know about the origins of the universe and the big bang, it is stated that it all started from an intensely hot and dense mass. This sounds like a singularity to me, which means a black ...
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1answer
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Based on our current observations, what all prevents a formed black from experiencing a repulsive force to overcome its gravitational force…? [duplicate]

Edit: I don't think this is a duplicate. [***] Question: Based on our current observations, what prevents a black from experiencing any sort of repulsive force to overcome its gravitational force ...
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Where does the energy of a photon trying to escape a black hole go?

I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its ...
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How are negative energy orbits around a Black Hole defined?

I have read several times that within the ergosphere or a Kerr Black Hole, it is possible to have particles that have an orbital energy value of less than 0. However, I do not understand the concept ...
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1answer
45 views

What effect would a singularity have on the fabric of space time?

Galaxies are always moving and it seems that there are SMBHs at the heart of most galaxies. If black holes actually twist up spacetime so much thru infinite density how do we know that after the ...
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1answer
78 views

If an object has a temperature, does it have to radiate?

I'm reading through a powerpoint presentation about Hawking Radiation (HR). They are explaining all of the reasons that built up to the postulate of HR, and one of the reasons is that if there is a ...
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Israel-Wilson-Perjés Solutions

I'm searching for a reference that gives explicitly the field strength (or at least the gauge fields) of the Israel-Wilson-Perjés Solution, using complex harmonic functions for the metric. In ...
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0answers
18 views

Hawking radiation and energy-negative energy pair production [duplicate]

A black hole evaporates through hawking radiation, what I don't get is the requirement for an energy-negative energy pair production. Since it's the black hole's gravitational energy that's ...
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30 views

Orbital period and velocity around a Kerr black hole relative to fixed stars

I've been trying to make progress on some of the smaller pieces of this question about the environment around a Kerr black hole. In order to calculate the effects of special relativistic Doppler shift ...
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1answer
87 views

Black holes shouldn't really exist? [duplicate]

General relativity states that for an observer sufficiently far from the gravitational field of a blackhole, the space time geodesic nearr the event horizon is so long that we should never observe an ...
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32 views

What if a black hole of normal matter and a black hole of antimatter collided? [duplicate]

This is a curiosity question. Considering same mass black holes. As nothing can come out of either black hole, would the annihilation result into a black hole of energy? Assuming the two black holes ...
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1answer
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How can black hole interior theories be falsified?

How can any theory of the interior of black holes be falsified if the interior cannot be observed?
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Why '1+log slicing condition' and 'Gamma Driver Shift Condition' were successful in black hole simulations?

The 1+log slicing and Gamma driver shift conditions are I want to know if there is a specific reason why these conditions were used most for Black Hole simulations in Numerical Relativty. And how ...
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154 views

Curvature Invariants in General Relativity and Singularities

Suppose that I want to check if a given metric is singular or not. I'm interested in curvature singularities, not coordinate singularities, so I can look to scalars made with Ricci, Riemann and Weyl ...
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1answer
58 views

Does a singularity appear the instant a black hole is formed? [duplicate]

Imagine a very heavy (tens of solar masses) star in its final moments before collapsing to form a black hole. The gravitational force exerted by the weight of the star overcomes the neutron degeneracy ...
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0answers
50 views

Is the general definition for black holes wrong? [duplicate]

I often read the definition of event horizon of a black hole as the region where the escape velocity is bigger than the speed of light. However this would imply that if you're inside the event horizon ...
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1answer
32 views

How do black holes join each other? [duplicate]

If black holes can swallow any object, and can radiate energy, then how could black holes join each other and can form bigger black holes?
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Can gravitational waves observed far from a black hole tell us anything about the multipole moments of a dynamical horizon?

In a paper by Ashtekar et al in 2013 on the approach to the final state to a stationary black hole they study the evolution of the multipole moments of dynamical horizons, which relax away (except for ...
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1answer
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Can you see external light approaching a black hole when you are within it?

We can't see a BH because light can't escape from it, but when you are within a BH (assuming you would stay alive) could you see any light from extrenal stars? If so, would the ligth be very bright or ...
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1answer
78 views

Does a black hole really slow down time?

When an object gets pulled into a black hole it seems to slow and stop, but could it be possibly be because the speed of light that hit the object and came back was slowing down as the object got ...
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Can a rotating black hole have a donut-shaped event horizon? [closed]

It is conjectured that a rotating black hole has at its center a ring-shaped singularity. Thus, at the center of the ring-shaped singularity the gravitational field must be zero (similar to ...
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1answer
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Information from inside a black hole

Now I'm hardly a physicist, but I am pretty interested in it. I was thinking about black holes and the movie Interstellar, and if you've seen it, then one of the central points about it is that they ...
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Is there any known dynamical process to obtain supertubes?

I'm looking for a dynamical process to produce supertubes. For example, microstates of the D1-D5 black hole system can be understood as supertubes. See, for example, ...
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35 views

Help needed to understand Kerr coordinate transformation

The (uncharged) Kerr metric for a black hole of mass $M$ and angular momentum $Ma$ takes the form $$ds^{2} = \Sigma\Big(\frac{dr^{2}}{\Delta} + d\theta^{2}\Big) + (r^{2} + a^{2})\text{sin}^{2}\theta ...
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Can you have black holes in your black holes?

Inspired by Are we inside a black hole?, can you have a black hole such that other black holes are in them? In particular, the event horizon of the larger black hole should completely enclose the ...
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1answer
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Is the question: 'Is our universe the inside of a black hole?' meaningful? [duplicate]

By meaningful I mean experimentally falsifiable.
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1answer
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Time-dependent time dilation for circular orbits around a Schwarzschild black hole?

Take two clocks, one in a circular orbit around a Schwarzschild black hole and another with a distant observer. The time dilation factor between the two clocks is said (at ...
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How is it possible to have such massive black holes? [duplicate]

Recent observations discovered really massive black holes, up to $20-40$ billions Solar masses. Now, according to an recent study and various computer simulations (I'm sorry, I don't have any ...
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2answers
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Wald's General Relativity, section 6.3 Page 144

I cannot understand how he reaches the conclusion in equation 6.3.36 and 6.3.37; even the terminology is somewhat confusing. This is a problem of bending of light under gravitational field. This is ...
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Power of blueshifted light falling on observer in circular orbit around Schwarzschild black hole

This answer explains that the time dilation for an observer in a circular orbit around a Schwarzschild black hole, relative to a distant observer at rest relative to the black hole, is given by the ...
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1answer
63 views

What Color Are Black Holes Really? (Yes, a serious question)

So I got into a mini-debate in science class today because I proposed that black holes aren't really black, they only look black because light can't reflect off them. But if you were to take the ...
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why does apparent horizon coincides with the event horizon in stationary spacetime?

Regarding the statement that the apparent horizon coincides with the event horizon (well, the intersection of the event horizon with the Cauchy surface where the apparent horizon is defined) in ...
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What would the effects of the GW150914 gravity wave burst be on observers much closer that 1.3B LY? [duplicate]

The effects of the GW150914 gravity wave burst were barely observable with state of the art instruments, i.e. LIGO. What would the effects of GW150914 gravity wave burst be on observers much closer ...
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Why didn't LIGO wait for a second observation of a gravitational wave? Are not reproducible results fundamental to science? [closed]

Wikipedia states, "Reproducibility is one of the main principles of the scientific method." So why did LIGO ignore a main principle of the scientific method? My whole life I have been taught that ...